1. Technical Field
The invention relates to a bellows actuation device, especially for robotic manipulator.
2. Description of the Background Art
Industrial robotic manipulators known in the art are mainly devised to handle objects having a hard structure and usually a good resistance, therefore they are usually well suited for industrial applications where a sure hold of hard and solid objects is required (e.g., metal pipes largely used in off-shore structures).
New requests are rising for robotic devices to be used, for example for scientific research activities (e.g., for biology, archaeology, geology, etc.), for example in the underwater field. This type of use requires the ability to handle and grasp fragile, delicate, and, often, soft objects, that are likely to be damaged if handled with conventional devices.
In order to prevent the possible damaging of the fragile objects, it could be possible to provide the manipulator with tactile sensors able to measure the force actually applied to the surface of the object, but this solution provides to be highly complex; and conspicuous technical problems must be addressed for its implementation in devices for special uses, as, for example the underwater one.
For example, a robotic manipulator of the articulated jaw type is known in the art and described in U.S. Pat. No. 4,607,998. The electrohydraulic actuation system subject of the patent is a two piston-cylinder assembly, fixed volume, hydraulic system operated by an electric motor by means of a screw device. Friction resulting from the piston seals and from the thrust bearings of the screw is necessary for the system operation, and is so tailored as to have an entity that restrains jaw motion due to applied external forces. This characteristic, while proving advantageous when the handled objects show a solid and hard structure, proves disadvantageous when the objects to be handled are fragile and soft ones, like biologic specimens, archaeological items, etc.
The object of the invention is to overcome the above drawbacks and to provide an actuation device using, for the active element, a flexible bellows operated by a pressurized fluid. The device of the invention solves also another problem in the case long and compliant bellows are used with pressures of the operating fluid of some relevance, according to the preferred embodiments of the invention. Bellows with these characteristics (i.e. usually having a high length-to-diameter ratio) have, starting from very low operating pressure, a structural instability phenomenon, named xe2x80x9cbucklingxe2x80x9d, causing an out-of-control sideways deformation and substantially reducing the useful output force as well as the range of motion, making this kind of bellows actually not suited to be used in manipulation devices.
The device of the invention comprises, in the basic embodiment, a bellows housed, for the most part of its length, inside a cavity, usually cylindrical in shape, having the inner diameter slightly larger than the outer diameter of the bellows so that the bellows is free to vary its length, but is restrained to bend sideways and therefore can be operated by a pressure fairly higher than the one that would cause the above mentioned instability phenomena in an unrestrained bellows. A specially suitable field of application of this type of actuator is the one of robotic grippers, where it provides more advantageous than solutions based on the use of other devices, like pneumatic or hydraulic piston-cylinder assemblies.
The bellows actuation device of the invention is virtually friction-free, thus allowing to obtain a relation between the operating pressure and the contact force applied to the handled object, that can be represented by a mathematical function without any discontinuity point. These characteristics make it possible to provide an accurate control of the contact force, thus allowing robotic manipulators able to handle fragile or delicate objects without damage.
The main advantage of the bellows actuator versus the piston-cylinder assembly is the absence of sliding seals and hence the absence of friction and wear.
This characteristic proves very useful for devices intended for handling fragile or delicate objects, by allowing an indirect measurement of the contact force applied to the object by means of a measurement of the operating pressure.
The manipulators that can be realized using the actuation device of the invention, characterized by being friction-free, overcome the above-said drawbacks and, when used in association to a suitable servo-control, can achieve a fine regulation of the contact force applied to the object, and hence a sure hold with no risk of damaging the object, without requiring complex tactile sensors.
Moreover, the manipulators subject of the invention have the characteristic of being very easy to implement and are suitable of being made of materials able to withstand fairly well the typical operating conditions of underwater robotics.
The main characteristics that are achieved are the followings.
No-friction: as well known, the friction-force vs. motion-speed relation is not only not-linear, but also not-continuous due to the transition between static and dynamic friction (when speed crosses the zero value). The presence of friction in a device that has to transfer force and motion between the actuating section and the actuated one of a manipulator, precludes the continuity of the relation linking the force applied to the object by the manipulator with the force produced by the driving device, and therefore does not allow to calculate the contact force applied to the object on the basis of the measurement of the force produced by the driving device.
Force Control: the feasibility to easily implement (without the use of special sensors) a force control (instead of a position control), makes it possible for the manipulator to have a degree of xe2x80x9ccompliancexe2x80x9d, that is the ability to adapt itself to the shape and to small displacements of the manipulated object with no substantial variation of the contact force.
A further advantage of the use of a bellows, is the feasibility to couple directly its ends to members of the manipulation device, each moving with respect to the others, with no need to use joints or couplings, because the bellows flexibility is usually adequate to allow the required degree of flexure.
Further objects and advantages of the invention will be clearly shown in the following detailed description of an example of embodiment and from the attached drawings, given for a non limiting explanatory purpose.